1. Field of the Invention
The present invention relates to a brushless motor used for rotating a recording disk such as CD and DVD and a disk drive device in which the brushless motor is installed.
2. Background Information
In general, a brushless motor for rotating a discoid recording disk comprises a fixing-side frame, a shaft rotatably supported by the fixing-side frame via a bearing, a rotor mounted on the shaft and rotated integrally with the shaft, a rotor magnet mounted on the rotor, and a stator supported by the fixing-side frame at a position opposing to the rotor magnet. Further, the rotor is provided with a turn table on which the disk is disposed, and the disk is rotated by rotating the rotor in the state in which the disk is disposed on the turn table.
The stator comprises a stator core having a plurality of teeth, and stator windings of, for example, three phases wound around the respective teeth of the core. The rotor magnet is formed in, for example, a cylindrical shape, and magnetized in a multipolar manner in such manner that different magnetic poles are circumferentially alternately arrayed. When a direction of a current distributed to the stator windings of the respective phases is changed, magnetic poles of the stator are rotated in one direction, which repeatedly generates attraction and repulsion relative to the magnetic poles of the rotor magnet. As a result, the rotor can obtain a rotating force.
In the foregoing constitution, as a means for detecting the magnetic poles of the rotor magnet and a rotational position of the rotor in order to change the current distribution with respect to the stator windings of the respective phases, a plurality of position detecting sensors (for example, three corresponding to the respective phases) such as Hall elements is disposed on the fixing-side frame in such manner that the respective sensors are shifted by a predetermined phase. Usually, the position detecting sensors are disposed on a circuit substrate provided on a lower side of the rotor magnet and secured to the fixing-side frame, and also disposed on a radially inner side relative to the rotor magnet and between the adjacent teeth of the stator core.
In recent years, a digital data processing speed has been increasingly advancing, in response to which the disk rotation has successfully attained a higher speed in the disk rotation motor of the foregoing type. As a recent trend, high-speed data recording and reproduction are realized by rotating the disk at such a high speed as, for example, approximately 12,000 rpm. However, it is contradictorily demanded to rotate the disk, which is installed in the high-speed disk rotation motor, at a low speed (for example, at most 100 rpm) using the motor. For example, in the case of rendering/printing data in various manners on a label surface (surface contrary to a recording surface) of the disk using a laser-type pickup device for recording and reproducing the data on the recording surface of the disk, it is necessary to rotate the disk at a low speed. However, it was almost impossible to respond to the demands that the low-speed rotation control and the high-speed rotation control are both satisfied by the rotation control of the conventional brushless motor in which only the position detecting sensors such as the Hall elements are provided.
A possible method in order to satisfy the foregoing demands which is conventionally employed is that the rotor is controlled to rotate at a high speed using the position detecting sensors such as the Hall elements, while an encoder is combined with a rotation shaft of the rotor so that a FG pulse required for controlling the low-speed rotation is generated from a rotation detection signal of the encoder and used to control the low-speed rotation. However, in the aforementioned method, the additional provision of the encoder results in complicating the structure and increasing costs. As a result, it becomes disadvantageously difficult to downsize the entire motor.